Snakewater, process, and standing are all complete wonders of playercount balance. They all play perfectly fine all the way up to 32 players. If anything, accusing valve of not caring about pub play on new maps is senseless at best.
I think valve explicitely looks for maps that work well at higher player counts.
For pub play with higher player counts matchmaking has what it needs. It has the somewhat minimal setting of 24p normal respawn. but also 32p insta respawn. If it would only accept maps for 6v6 you would give players a worse experience.
Where at 6v6 its normaly comp and just gets arranged by the league itself. So custom maps that arent in the game are not a problem here at all.
If i look at upward and barnblitz. both dont feature that much single path areas. If they do its most of the time at the final capture point, and even then its often still 3 entrances. For comp 2 would have been acceptable because the player count is often just too low for extra, but on pub this is devestating as it becomes a turtlefest.
If i would look at upward i notice alot of possible entrances to the final area. And they are most of the time only needed for pub. In a comp game when 4 people died you often can push the cart far enough anyway. Its still more based on making player kill than to perform the objective.
In a 5cp map for pub you dont want spires to capture. captures are hard enough already with more defense. But for comp having a flat ground is simply too easy. The spire there is to slow down the pace by making a defend action far more effective.